DESCRIPTION (Taken from the application): Injured and repaired tendons and reconstructed ligaments have impaired mechanical properties. They lose half of their strength and stiffness, which may lead to re-injury or to long-term joint problems. The causes of the decreased tendon mechanical properties are largely unknown, but may be related to disruption in the mechanical integrity of tendon micro-structures such as the fibers, matrix or fiber-matrix interactions. Even in normal undamaged tendon, little is known of the mechanical contributions of these micro-structures. In this study, it is hypothesized that the mechanical contributions of the tendon micro-structures of fiber, matrix, and fiber-matrix interactions are significant in normal tendon behavior. In the current proposal, engineering tools of material modeling and tensile testing will be used to test this hypothesis. While directly measuring tendon micro-structure is clearly valuable, our approach is to begin by representing these micro-structures in a mathematical model and to add targeted biochemical and micro-structural measures in subsequent studies. The following specific aims will be completed. In Specific Aim 1 tendon micro-structure will be represented in a material model. We have previously represented the micro-structure of the anulus fibrosus of the spine in a material model, which resulted in new insights into the role of fiber-matrix interactions in that tissue. In the model, structural tensors will be used to mathematically represent the tendon fiber within an isotropic matrix. In Specific Aim 2 the mechanical contribution of tendon micro-structures will be measured in tensile tests. Finally, in Specific Aim 3 the model will be validated for complex loading by predicting and directly measuring the stress-strain behaviors in biaxial tension. By quantifying the mechanical contributions of these micro-structures in normal tendon, we will not only obtain insight into normal tendon function, but also generate baseline information from which to study questions of where and how tendon mechanical properties are impaired following tendon repairs or ligament gratis. This information will enable us to develop and assess potential treatment modalities.